Since I just replaced a Samsung Galaxy S6 screen yesterday, it's probably the touch sensor button cable which is connected to the charger port flex assembly. Sounds like you've cut or damaged the flex cable on accident.

So home button and finger print work, but the back button does not work at all and the menu button works and lights up?

Anyway, the charger port assembly has two separate cables going to each corner for each touch sensor.

TL:DR - Yes, a daughter-board replacement will fix this issue (Charger port flex assembly). Problem is that you will need to take out the screen again.

Next time if you do open it, you will need to start lifting up the screen from top then to bottom. Then you can separate the touch sensors being glued to the glass by using a spudger between them.